How to fix blank space on the bottom of lighting HDRs
What you are seeing is blank space not stored in the HDR. Since the lighting received from the ground in real life is actually quite minimal, this information is not included in order to save memory. Usually you will have an environment providing those reflections in your scene instead. 

To get around this issue, you can do a few things:


Change the camera angle
Rotate everything away from the horizon
Add some horizon elements
Use a different HDR


Camera Angle:


Move the camera so that the black is no longer visible

Rotate Objects:


Just rotate the whole scene so it is angling towards the camera

Horizon Elements:


These could be mountains, trees, hills, etc.



Now you mention that you do not like the camera angle when moved very low, and I understand that. Really your only option is to (if you have the purchased version) add a different HDR, or add some horizon elements like trees, mountains, etc. A scene that consists of just a house on a hill can be somewhat amateurish, as beginners do not like to add nature to their architecture renders. 

Do a little more research, and you will find that almost all the powerful Arch Viz images are closer, and have convincing surrounding elements. I think that maybe this issue will help push you to explore more creative angles and environment choices. Such will only result in a more powerful result.

If you still want to use your original idea, I would recommend making whatever would be on the other side of the valley, or over the hill. Having some atmospheric fading distance wise will really help sell the effect, making it more believable that it is on a hilltop.